Ok, myself just like everyone one else is all of a sudden not able to take transactions with the authorize.net AIM module. Mine was working for 6 months, never a problem until last month April 26th, around 1pm. And now I get the same error as everyone else.
I even installed the latest AIM module and no go. Here is the deal.. Authorize.net has been making some changes on their end and have been migrating user accounts with the new changes and killing your AIM module.
Why do you think there is a hour wait on the phone when you call them and a 2 hour wait in the live help? All they tell you is to consult the developers guide. They know what is going on there and I have now had to enable my paypal account to take sales. Only problem with paypal is that they sometime reject transactions for no reason on good card sales.
So where do we go from here... You got me.. I am not skilled enough in php / the AIM module to fix it. I hope someone can figure this out because I am done with authorize.net
Just my two cents and know you know what it going on. These kind of things don't happen out of the blue unless something has charged and it did on on the side of authorize.net.